The present invention relates to a lubricating oil composition and more particularly to a multi grade lubricating oil composition which is excellent in shear stability.
One embodiment of the present invention relates to a multi grade engine oil composition and more particularly to a multi grade engine oil composition which is excellent in shear stability and engine cleanliness. This multi grade engine oil composition can be used as an internal combustion engine oil for a gasoline engine, a diesel engine, a gas engine and other special engine, and further as a compressor oil. Another embodiment of the present invention relates to a high viscosity index lubricating oil composition containing a mineral oil as a major component, and more particularly to a lubricating oil composition which has a particularly high viscosity index, is excellent in shear stability and further in extreme pressure properties and anti-wear properties, and thus which can be used as an oil for cars and industrial gears, a power stearing oil, a tractor oil, a shock absorber oil, a hydraulic fluid, a door check oil, a bearing oil and so on.
The conventional multi grade engine oils have a disadvantage in that their viscosity is markedly decreased by mechanical shear applied thereto during their use, because they contain a relatively large amount of a polymer having a greatly high molecular weight as an agent to improve viscosity-temperature characteristics (a viscosity index improver). Particularly under high oil temperatures, the reduction in viscosity is great and the problem of abrasion of bearing metal often occurs. Moreover, addition of a large amount of the polymer leads to a reduction in engine cleanliness and particularly, to increase the engine deposits.
On the other hand, a mineral oil with high molecular weight polymers compounded thereto has heretofore been used as a high viscosity index lubricating oil.
However, since this lubricating oil contains a relatively large amount of high molecular weight polymers, its shear stability is seriously poor; when subjected to mechanical shear, it suffers from disadvantages in that viscosity is markedly decreased, initial performance cannot be satisfied, and abrasion is increased. Thus the lubricating oil is unsuitable for practical use.